Chap. V. CONCLUDING REMARKS. 193 



bearers if the trunks are branched or bear twigs, and 

 by some species if the bark is rugged. 



The advantage gained by climbing is to reach the 

 light and free air with as little expenditure of organic 

 matter as possible ; now, with twining plants, the stem 

 is much longer than is absolutely necessary; for 

 instance. I measured the stem of a kidnev-bean, which 

 had ascended exactly two feet in height, and it w r as 

 three feet in length : the stem of a pea, on the other 

 hand, which had ascended to the same height by the 

 aid of its tendrils, was but little longer than the height 

 reached. That this saving of the stem is really an 

 advantage to climbing plants, I infer from the species 

 that still twine but are aided by clasping petioles or 

 tendrils, generally making more open spires than 

 those made by simple twiners. Moreover, the plants 

 thus aided, after taking one or two turns in one direc- 

 tion, generally ascend for a space straight, and then 

 reverse the direction of their spire. By this means 

 they ascend to a considerably greater height, with the 

 same length of stem, than would otherwise have been 

 possible ; and they do this with safety, as they secure 

 themselves at intervals by their clasping petioles or 

 tendrils. 



We have seen that tendrils consist of various organs 

 in a modified state, namely, leaves, flower-peduncles, 

 branches, and perhaps stipules. With respect to 

 leaves, the evidence of their modification is ample. 

 In young plants of Bignonia the lower leaves often 

 remain quite unchanged, whilst the upper ones have 



